How to Make It
Light a grill or preheat a grill pan. In a large skillet, heat 2 tablespoons of the olive oil. Add the onions, season with salt and cook over moderately high heat until softened and browned, about 8 minutes. Add the chipotle powder and cook for 30 seconds. Add the oregano and the vinegar and cook for 1 minute. Transfer to a plate.
Meanwhile, in a bowl, combine the mango, cucumber, serrano, lime juice and cilantro and season the salsa with salt.
Oil the grill. Rub the tuna with the remaining 1 tablespoon of olive oil and season with salt. Grill over high heat until lightly charred and pink within, about 3 minutes per side. Transfer to a plate. Grill the tortillas until hot and soft, about 10 seconds. Stack the tortillas and wrap them in foil to keep warm.
Thinly slice the tuna and transfer to a platter. Serve with the onions, salsa and tortillas. Let everyone make their own tacos.